Stabilized nitric acid



ilnited rates Fatent 3,l9,@8l Patented Jan. 30, 1962 tice This invention relates to stabilized nitric acid. In one aspect this invention relates to the stabilization of concentrated nitric acid. In another aspect this invention relates to a stabilized oxidizer mixture comprising nitric acid and a quaternary ammonium compound.

Concentrated nitric acids such as are sold in the form of ordinary concentrated acid, white fuming nitric acid (WFNA), red fuming nitric acid (RFNA), and anhydrous nitric acid are important commercial products. These materials have many important uses such as for the preparation of fertilizers, in cleaning and etching solutions, etc. In recent years said concentrated nitric acids have been used as oxidizer ingredients for rocket propellants, in both bipropellant applications and in monopropellant applications. When used as the oxidizer component in a bipropellant system the acid is carried in the rocket or missile in a separate compartment or tank and is injected into the combustion chamber of the rocket motor as a separate stream where it contacts the fuel component, also injected as a separate stream. Said fuel component can be a hypergolic fuel, such as aniline, in which case spontaneous ignition takes place upon contact between the acid and thehypergolic fuel. Said fuel can also be a non-hypergolic fuel in which case the mixture formed when the acid and fuel are contacted is ignited by any suitable means such as a spark igniter. Monopropellant compositions consisting essentially of a fuel component such as an amine nitrate dissolved in nitric acid represent another application for said acids. In this type of application the monopropellant is stored in a tank carried by the rocket and is injected into the combustion chamber of said rocket where it is ignited.

Nitric acids, particularly concentrated nitric acids are not stable during storage, and tend to decompose according to the reaction:

Such decomposition of nitric acid during storage not only decreases the oxidizing power of the acid but also builds up pressure in the storage vessel. Over a period of time gas pressure can build up in the storage vessel to a point which constitutes a serious hazard, and can sometimes even result in rupture of the storage Vessel. Even when the acid is stored in a vented vessel so that storage pressure is of no concern, the loss in product quality which occurs upon prolonged storage is serious. Thus, it is important for all uses of nitric acid that, if possible, said acid be stabilized to reduce decomposition during storage. Stabilization is particularly important where the acid must be stored in closed containers, such as in the fuel tanks in a rocket motor, etc.

We have discovered that quaternary ammonium compounds are eifective stabilizers for nitric acid. Thus, broadly speaking, the present invention resides in a stabilized nitric acid containing a small but effective amount of a quaternary ammonium compound.

An object of this invention is to provide a stabilized nitric acid. Another object of this invention is to provide an improved oxidizer mixture which is suitable for use in reaction motors, such as rocket motors, etc. Still another object of this invention is to provide a stabilized nitric acid which can be safely stored in closed containers. Other aspects, objects, and advantages of the invention will by their reaction With strong bases.

be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided a stabilized nitric acid consisting essentially of nitric acid and from about 0.1 to about 10 weight percent of at least one quaternary ammonium compound characterized by a formula selected from the group consisting of wherein: each R is selected from the group consisting of alkyl, cycloalkyl, alkenyl, and cycloalkenyl radicals containing from 1 to 8 carbon atoms wherein when more than one carbon atom is present the carbon atom attached to the quaternary nitrogen atom is attached to adjoining carbon atoms by single valence bonds, and said radicals substituted with'a substituent selected from the group consisting of nitro, nitrato, and cyano radicals, the total number of said substituents in said R groups being not greater than one-half the total number of carbon atoms in said R groups; R is selected from the group consisting of alkylene radicals containing from 1 to 12 carbon atoms, and alkenylene radicals containing from 4 to 12 carbon atoms wherein the carbon atom attached to the quaternary nitrogen atom is attached to adjoining carbon atoms by single valence bonds; X is an anion selected from the group consisting of nitrate, perchlorate, monohydrogen phosphate, dihydrogen phosphate, hydrogen sulfate, sulfate, hydroxyl, orthoborate, dihydrogen borate, and tetraborate anions; a and b are each integers of from 1 to 4, and the product of a multiplied by the number of quaternary nitrogen atoms is equal to the product of b multiplied by the valence of said anion X.

It is to be noted that the compounds used in the practice of the invention are quaternary ammonium compounds as distinguished from acid salts of amines. The two classes of compounds are distinctly difierent chemically. As used herein and in the claims the term quaternary ammonium compound refers to a compound containing at least one nuclear nitrogen atom having its five so-called valence bonds attached toatoms (or radicals) other than a hydrogen atom. For example, such as four nitrogen to carbon bonds and one nitrogen to nitrate (or other anionic radical) bond. In other words, the available valence bonds of said nuclear nitrogen atom are satisfied by other than a hydrogen radical. For example, consider the two compounds (1) trimethylamine nitrate and (2) tetramethyl ammonium nitrate which have the formulas (CH NHNO and (CH NNO respectively. It will be noted that in the acid salt trimethylamine nitrate one of the valence bonds of the nuclear nitrogen atom is satisfied by a hydrogen atom whereas in the quaternary ammonium compound tetramethyl ammonium nitrate all five valence bonds are satisfied by radicals other than hydrogen.

The two classes of compounds can also be distinguished v For example, when an acid salt such as trimet-hylamine nitrate is reacted with sodium hydroxide the amine is liberated along with the formation of sodium nitrate and water; whereas a quaternary ammonium compound such as tetramethyl ammonium nitrate has no hydrogen available for the formation of water and there is obtained the tetramethyl ammonium hydroxide [(CH NOH] and sodium nitrate, and no free amine is liberated.

Examples of quaternary ammonium compounds suitable for use in the practice of the invention include, among others, the following:

Tetramethylammonium nitrate Tetramethyla-mmonium perchlorate Tetramethylammonium hydrogen sulfate Tetraethylammonium nitrate Triethylmethylammoniurn hydrogen sulfate Triethylmethylammoniurn nitrate Triethylmethylammonium perchlorate Triethylmethylammonium dihydrogenphosphate Diethyldimethyla-mmonium nitrate Ethyltrimethylammonium nitrate Diallyldimethylammonium nitrate Tetra-n-propylammonium nitrate Tetra-n-propylammonium dihydrogen phosphate Tetra-n-propylammonium hydrogen sulfate Di(2-methylhexyl) dimethylammoniurn perchlorate Tetra-n-butylammonium nitrate Tetra-n-butylammonium perchlorate Trimethylpenty-lammonium perchlorate Ethyltripentylammonium perchlorate Dihexyldi-n-propylammonium nitrate Ethyltri-n-octylammonium nitrate Tetra-n-octylammonium nitrate Methyltri-n-octylammonium nitrate Methyltri-tert-octylammonium nitrate N,N,N',N-tetramethyl diethylenediammom'um dinitrate N,N,N',N'-tetra-n-butyl diethylenediammonium din-itrate Trimethylcyclohexylammonium nitrate Trimethylcyclohexylammonium metaborate Allyltrimethylammonium nitrate Triethyl-Q-methallylammonium nitrate 3-hexenyltriethylammouium nitrate Di-3-hexenyldimethylammonium nitrate Ethyldimethyl-Z-cycloocteneylammonium nitrate Allyldiethylcyclohexylammonium perchlorate N,N,N,N',N',N'-hexamethyl methanediammonium dinitrate N,N,N,N',N,N'-hexamethy1 methanediammonium diperchlorate N,N,N,N',N',N'-'hexaethyl methanediammonium diperchlorate N,N,N,N,N',N-hexamethylethane-1,Z-diammonium dinitrate N,N,N,N,N',N-hexamethylpropane-1,B-diammonium dinitrate N,N,N,N',N,N-hexamethylbutane-1,4-diammonium dinitrate N,N,N,N',N,N-hexamethylbutane-1,4-diammoniurn diperchlorate N,N,N,N',N',N hexarnethylbutane-14-bis(diammonium hydrogen phosphate) N,N,N,N',N',N-hexamethylbutane 1,4 bis(ammonium hydrogen sulfate) N,N,N,N,N,N-hexa-n-propylbutane1,4-diammonium dinitrate N,N,N,N,N,N'-hexamethyloctane-1,S-diammonium dinitrate l N,N,N,N',N,N' hexamethyldodecane-l,IZ-diammonium dinitrate N,N,N,N',N',N-hexa(2 nitratoethyl)propane-1,3-diammonium dinitrate N,N,N,N',N,N-hexa(2 cyanoethyl)hexane-1,G-diammonium dinitrate N,N,N,N',N,N'-hexa(2 cyanoethyl)hexane-1,4-diamrnonium dinitrate N,N,N,N',N',N'-hexamethy1(2 butene)-i1,4-diammonium dinitrate N,N'-dimethyltriethylene diammonium dinitrate N,N-dimethyltriet-hylene diammonium diperchlorate N,N-diethyltriethylene diammonium dinitrate N-ethyl-N-methyltriethylene diammonium dinitrate N,N-diisopropyltriethylene diammonium dinitrate N,N'-dicyclohexyltriethylene diammonium dinitrate N,N,N,N',N,N-hexamethyl(4 dodecene)-1,l2-diammonium dinitrate N,N,N,N,N',N-hexaethyl(2,7 diethyl-4-octene)-l,7-diammonium dinitrate Tetramethyl ammonium tetraborate Ethyltrimethyl ammonium orthoborate Triethylmethyl ammonium perborate Tetramethylammonium hydroxide Tetraethylammonium hydroxide Tetra-n-propylammoniurn hydroxide Trimethylpentylammonium hydroxide Methyltri-n-octylammonium hydroxide N,N,N,N',N,N'-hexamethylmethane droxide N,N,N,N,N,N' hexaethyloctane 1,8 diammonium hydroxide diammonium hy- Ihe quaternary ammonium compounds used in the practice of the invention can be prepared by any of a number of suitable methods well known to those skilled in the art. A first method comprises in general, the addition of a suitable alkyl chloride to a suitable amine which has been dissolved in a highly polar solvent such as nitrobenzene. The choice of the alkyl chloride, the amine, and the specific reaction temperature and quantities of reactants and solvents will of course depend upon the particular compounds it is desired to prepare as will be understood by those skilled in the art. Methyl alcohol can be used as a co-solvent to increase the solubility of the alkyl chloride in the polar solvent, if desired. After reaction of said alkyl chloride and said amine, the resulting quaternary ammonium chloride is separated and dried under vacuum. Said quaternary ammonium chloride is then dissolved in methyl alcohol and the exact percent of the chloride salt in solution is determined gravimetrically by means of silver chloride precipitation.

The thus obtained standardized alcoholic quaternary ammonium chloride solution is then treated with a stoichiometric amount of standardized alcoholic potassium hydroxide solution and the resulting potassium chloride which precipitates (essentially quantitative) is filtered. The filtrate is then treated with approximately a 10% excess of the acid corresponding to the quaternary ammonium salt to be prepared, e.g., nitric acid for nitrates, perchloric acid for perchlorates, etc. After the acid addition is complete, the mixture is evaporated to near dryness on a steam bath. The mushy residue is then dissolved in a minimum amount of isopropyl alcohol and the quaternary ammonium salt precipitates as a white crystalline product upon the addition of ethyl ether. The precipitated quarternary ammonium salt is filtered, washed with more ether and dried in a vacuum desiccator. This procedure has been found reliable and the quarternary ammonium salts so prepared are normally obtained in yields of -98 weight percent.

A second method comprises treating trialkylamine with a solution of an alkyl nitrate in ethyl ether and allowing the mixture to stand for several days, after which the solvent is evaporated.

A third method comprises treating a suitable amine with a suitable alkyl halide and then reacting silver nitrate with the resulting quaternary ammonium halide to produce the corresponding quaternary ammonium nitrate. Tetramethylammonium nitrate was prepared by this method.

The quaternary ammonium compounds are effective as stabilizers for nitric acids when used in small amounts.

Generally, the amount used to stabilize the nitric acid is in the range of 0.1 to percent, preferably within the range of about 0.5 to 5 percent, by weight of the total solution. The invention is applicable for stabilizing nitric acid of any concentration ranging from dilute acids containing as little as 1 weight percent HNO up to and including anhydrous acids containing essentially 100 weight percent HNO However, since the more concentrated acids are more unstable, the invention finds its greatest use in stabilizing the more concentrated acids, particularly those containing at least about 50 weight percent, or more, HNO The invention is particularly applicable for stabilizing the concentrated white fuming nitric acids, red fuming nitric acids, and anhydrous nitric acid which are available commercially. White fuming nitric acid usually contains about 90 to 99 weight percent HNO from 0 to 2 weight percent N0 and up to about 10 weight percent water. Red fuming usually contains about 70 to 90 weight percent HNO from 2 to 25 weight percent N0 and up to about 10 weight percent water. Fuming nitric acids, both white and red, having concentrations of water, N0 and HNO difierent from those given can of course be stabilized in accordance with the invention.

In the practice of the invention the acid to be stabilized can be mixed with one or more of the above-described quaternary ammonium compounds in any convenient manner. It is generally preferred to add the quaternary ammonium compound to the acid at temperatures below about 50 C., e.g., 0 to 30 C., with good agitation. The resulting acid solutions have an enhanced stability as measured, for example, by measuring changes in pressure in a closed container containing the acid solutions in storage.

The following example will serve to further illustrate the invention.

Example Anhydrous nitric acid was prepared by distilling red fuming nitric acid in the presence of sulfuric acid. Analyses of typical batches of a distilled anhydrous nitric acid showed that the product contained more than 99.8 weight percent HNO and less than 0.2 weight percent oxides of nitrogen.

The effectiveness of the qua-ternary ammonium compounds as stabilizing agents for nitric acid is shown by a comparison of the storage stability at 200 F. of a sample (9.9 grams) of said stabilized acid containing 0.1 gram of tetramethyl ammonium nitrate with another sample of anhydrous nitric acid containing no stabilizing agent. The thermal stability of said two samples was determined according to the following procedure.

A small glass tube constructed from one-fourth inch I.D. glass pipe, which will withstand a pressure greater than 1,000 p.s.i., is filled about two-thirds full (about 6 milliliters) with the nitric acid to be tested. Said tube is fitted with a safety head containing a blowout disc which will rupture at about 200 p.s.i. pressure. The small glass bomb is then placed in a constant temperature bath containing cold water and is connected to a pressure recorder and to a supply of compressed nitrogen gas. The pressure in said bomb is then increased to about 75 p.s.i.g. with nitrogen to check the system for leaks and, after checking, the pressure in said bomb is reduced to p.s.i.g. The temperature in the constant temperature bath, which can be regulated to maintain a temperature of 200 F., is increased and the time at which a temperature of 200 Fa is reached is taken as the start of the test. The test is terminated when the pressure in said bomb exceeds 100 p.s.i., or when the blowout disc is ruptured (the pressure rise is often rapid after 100 p.s.i. is reached). The storage life of the acid being tested is recorded as the time necessary for the'pressure in said bomb to increase from 20 to 100 p.s.i. at a temperature of 200 F.

The results of the above tests showed that the pressure in the bomb containing the acid stabilized with tetramethyl ammonium nitrate increased slowly to 75 p.s.i.g. during the first 15 hours of the test and then decreased slowly to 33 p.s.i.g. during the succeeding 454 hours after which the test was terminated. Thus the stabilized acid had a storage life greater than 469 hours. The pressure in the bomb containing the sample of unstabilized nitric acid increased to 61 p.s.i.g. in 9 hours and the test was terminated at 28 hours when the pressure in said bomb had reached p.s.i.g. Thus, the unstabilized acid had a storage life of only 28 hours.

The above tests show that the ntric acid stabilized with tetramethyl ammonium nitrate had a storage life more than sixteen times longer than the unstabilized nitric acid.

Since many possible embodiments can be made of this invention without departing from the scope thereof, it is to be understood that all matter herein set forth is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. A stabilized nitric acid consisting essentially of nitric acid and from about 0.1 to about 10 weight percent of at least one quaternary ammonium compound characterized by a formula selected from the group consisting of wherein: each R is selected from the group consisting of alkyl, cycloalkyl, alkenyl, and cycloalkenyl radicals containing from 1 to 8 carbon atoms wherein when more than one carbon atom is present the carbon atom attached to the quaternary nitrogen atom is attached to adjoining carbon atoms by single valence bonds, and said radicals substituted with a substituent selected from the group consisting of nitro, nitrato, and cyano radicals, the total number of said substituents in said R groups being not greater than one-half the total number of carbon atoms in said R groups; R is selected from the group consisting of alkylene radicals containing from 1 to 12 carbon atoms, and alkenylene radicals containing from 4 to 12 carbon atoms wherein the carbon atom attached to the quaternary nitrogen atom is attached to adjoining carbon atoms by single valence bonds; X is an anion selected from the group consisting of nitrate, perchlorate, monohydrogen phosphate, dihydrogen phosphate, hydrogen sulfate, sulfate, hydroxyl, orthoborate, dihydrogen borate, and tetraborate anions; a and b are each integers of from 1 to 4, and the product of a multiplied by the number of quaternary nitrogen atoms is equal to the product of b multiplied by the valence of said anion X.

2. A stabilized nitric acid according to claim 1 wherein the nitric acid to be stabilized contains at least 50 weight percent HNO 3. A stabilized nitric acid according to claim 1 wherein the nitric acid to be stabilized contains at least 70 weight percent HNO 4. A stabilized nitric acid according to claim 2 wherein the amount of said quaternary ammonium compound is within the range of about 0.5 to 5 weight percent.

5. A stabilized nitric acid according to claim 3 wherein 7 the amount of said quaternary ammonium is within the range of 0.5 to about 5 weight percent.

6. The stabilized nitric acid of claim 1 wherein said quaternary ammonium compound is N,N-dimethyltriethylenediammonium dinitrate.

7. The stabilized nitric acid of claim 1 wherein said quaternary ammonium compound is N,N'-diethyltriethylenediammonium dinitrate.

8. The stabilized nitric acid of claim 1 wherein said quaternary ammonium compound is N-ethyl-N'-methyltriethylenediammonium dinitrate.

9. The stabilized nitric acid of claim 1 wherein said quaternary ammonium compound is tetramethylammonium nitrate.

10. The stabilized nitric acid of claim 1 wherein said quaternary ammonium compound is tetraethylammonium nitrate.

11. The stabilized nitric acid of claim 1 wherein said quaternary ammonium compound is N,N,N,N,N',N'- hexamethylpropane-1,3-ammonium dinitrate.

12. The stabilized nitric acid of claim 1 wherein said quaternary ammonium compound is N,N,N,N',N,N- hexamethylbutane-1,3-ammonium dinitrate.

Cole -Q Apr. 21, 1936 Beiswanger Nov. 26, 1951 

1. A STABILIZED NITRIC ACID CONSISTING ESSENTIALLY OF NITRIC ACID AND FROM ABOUT 0.1 TO ABOUT 10 WEIGHT PERCENT OF AT LEAST ONE QUATERNARY AMMONIUM COMPOUND CHARACTERIZED BY A FORMULA SELECTED FROM THE GROUP CONSISTING OF 